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The eighth term of a pair of rational recurrences

Proved
WorkbookSource.problem_33900

by wamlart · Sep 12, 2026 · Mathlib 0df444a (Lean v4.33.1)

lean-workbooksource-checked

Define two sequences of rational numbers as follows: let a0=2a_0 = 2a0​=2 and b0=3b_0 = 3b0​=3 , and recursively define an=an−12bn−1a_n = \frac{a_{n-1}^2}{b_{n-1}}an​=bn−1​an−12​​ and bn=bn−12an−1b_n=\frac{b_{n-1}^2}{a_{n-1}}bn​=an−1​bn−12​​ . Find b8b_8b8​ , leaving your answer in the exponential form mn/pqm^n/p^qmn/pq .

Source: InternLM Lean-Workbook, record lean_workbook_33900 (Apache-2.0). The complete source proposition and its explicit variable declarations are preserved.

Preamble
import Mathlib
open Real Nat
Formal statement
theorem WorkbookSource.problem_33900 (b : ℕ → ℚ) (a : ℕ → ℚ) (b0 : b 0 = 3) (a0 : a 0 = 2) (hb : ∀ n, b (n + 1) = (b n)^2 / a n) (ha : ∀ n, a (n + 1) = (a n)^2 / b n) : b 8 = (3^3281)/2^3280  :=  by sorry
Source
https://huggingface.co/datasets/internlm/Lean-Workbook/blob/main/lean_workbook.json, record lean_workbook_33900; Apache-2.0

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